If the document element has opacity or has a background color with alpha, then stale pixels in the viewport can show through.
Created attachment 26162 [details] Testcase Testcase. Fix should be similar to bug 22938.
Actually, the problem does not occur with background-color. Only opacity seems to be problematic.
Created attachment 26231 [details] Patch, testcase, changelog
Committing to http://svn.webkit.org/repository/webkit/trunk ... M LayoutTests/ChangeLog A LayoutTests/fast/backgrounds/opacity-on-document-element.html M LayoutTests/fast/transforms/identity-matrix.html A LayoutTests/platform/mac/fast/backgrounds/opacity-on-document-element-expected.checksum A LayoutTests/platform/mac/fast/backgrounds/opacity-on-document-element-expected.png A LayoutTests/platform/mac/fast/backgrounds/opacity-on-document-element-expected.txt M LayoutTests/platform/mac/fast/transforms/identity-matrix-expected.txt M WebCore/ChangeLog M WebCore/rendering/RenderView.cpp Committed r39463